Research and development works have long been made on solid electrolytes. Well known in the art are inorganic materials, for example, .beta.-alumina, Li.sub.2 TiO.sub.3, RbAg.sub.4 I.sub.5, silver iodide, and phospho-tungstate. However, inorganic materials have many drawbacks including (1) a high specific gravity, (2) difficulty to form to various shapes, (3) difficulty to form a flexible thin film, and (4) low ionic conductivity at room temperature. These drawbacks must be overcome before the inorganic materials can be used in practice.
In the recent years, organic materials drew attention as a substitute capable of overcoming the above-mentioned drawbacks. Such organic material are commonly formulated as solid polymer electrolytes (SPE) comprising a polymer serving as a matrix such as a polyalkylene oxide, silicone rubber, fluoro-resin or polyphosphazene and an electrolyte serving as a carrier such as LiClO.sub.4 or LiBF.sub.4 admixed and dissolved therein. As compared with the inorganic materials, these SPE's are lightweight, flexible and easy to form or work into film. For these years, active research and development efforts have been made to formulate more practical SPE while maintaining these advantages.
The applied field of SPE is generally classified into
(1) commercial small-size secondary batteries with the features of room temperature operation and low output and PA0 (2) large-size secondary batteries with the features of high-temperature operation and high output. The small-sized secondary batteries (1) use as a separator a gel SPE in which a low-boiling aprotic organic electrolytic solution is absorbed and carried by a polymer for improving ionic conduction. Partially because their battery construction is substantially the same as lithium ion batteries, they have already reached a practically acceptable level as a low-output, compact, ultra-thin battery. PA0 methoxyethylene glycol mono(meth)acrylate, PA0 methoxypolyethylene glycol mono(meth)acrylate, PA0 octoxypolyethylene glycol-block-polypropylene glycol mono(meth)acrylate, PA0 lauroxypolyethylene glycol mono(meth)acrylate, PA0 stearoxypolyethylene glycol mono(meth)acrylate, PA0 allyloxypolyethylene glycol mono(meth)acrylate, PA0 nonylphenoxypolyethylene glycol mono(meth)acrylate, PA0 nonylphenoxypolypropylene glycol mono(meth)acrylate, PA0 nonylphenoxypoly(ethylene glycol-propylene glycol)mono(meth)acrylate, PA0 ethylene glycol di(meth)acrylate, PA0 polyethylene glycol di(meth)acrylate, PA0 propylene glycol di(meth)acrylate, PA0 polypropylene glycol di(meth)acrylate, PA0 polyethylene glycol-block-polypropylene PA0 glycol-block-polyethylene glycol di(meth)acrylate, PA0 polytetramethylene glycol di(meth)acrylate, PA0 poly(ethylene glycol-tetramethylene glycol)di(meth)acrylate, and PA0 poly(propylene glycol-tetramethylene glycol)di(meth)acrylate.
The class (2) includes lithium polymer batteries which are contemplated to use lithium metal as the negative electrode and expected to find application in electric automobiles and overnight power storage systems in the near future. In these large-size secondary batteries, however, a substantial quantity of heat is generated upon charging/discharging cycles and the battery itself is considerably increased in temperature. If gel type SPE is used as in (1), an envelope can of the battery can be bulged due to the vapor pressure of the electrolytic solution, and even the danger of explosion at worst is pointed out. To solve these problems, large-size secondary batteries of the high-temperature operation type were proposed in which a dry SPE system is heated to a temperature of 60 to 80.degree. C. for providing an increased ionic conductivity. On these batteries, research and development efforts have long been made mainly in US and Europe. However, there is not available at present the SPE which is fully safe, has improved film strength and generates no vapor pressure within the battery system even when exposed to such a high-temperature region.
Research reports regarding (2) include M. Watanabe et al., Solid State Ionics, 79 (1995), 306-312, and S. Kohjiya et al., Second International Symposium on Polymer Electrolytes, ed. by B. Scrosati, Elsevier, Appl. Sci., London (1990), pp. 187-196. Despite an ionic conductivity reaching the order of 10.sup.-4 S/cm near room temperature, these SPE's are unacceptable in practice because of the lack of film strength.
In Japanese Patent No. 1,842,047 (Japanese Patent Publication No. 5-51612), Makromol. Chem. Macromol. Symp., 25 (1989), 249, Reactive and Functional Polymers, 37 (1998), 169-182, and J. Polym. Sci., Part A: Polym. Chem., 36 (1998), 3021-3034, the same assignee as the present invention proposed how to synthesize a block-graft copolymer, which becomes a model leading to the present invention.
In order to utilize the block-graft copolymer as an ion conducting solid, Japanese Patent No. 1,842,048 (Japanese Patent Publication No. 5-51632) proposes as SPE a composition comprising a block-graft copolymer in admixture with an inorganic salt containing at least one element selected from among Li, Na, K, Cs, Ag, Cu and Mg in an amount of 0.05 to 80 mol % based on the alkylene oxide units in the copolymer. JP-B 5-74195 discloses a Li battery in which a composite of a similar block-graft copolymer with a Li ion salt is incorporated as the electrolyte. However, these proposals failed to reach the practically acceptable level because of low ionic conduction at room temperature.
For improving the ionic conductivity at room temperature, the assignee proposed in JP-A 3-188151 a block-graft copolymer composition comprising in admixture, a composite of a block-graft copolymer with an inorganic ion salt and a polyalkylene oxide. It was found that if a large amount of polyalkylene oxide is added to the block-graft copolymer, the polystyrene domain serving to maintain mechanical strength is partially dissolved so that the film is weakened.
To solve the newly derived problem, the assignee developed a block-graft copolymer which is insoluble in various polyalkylene oxides and has silyl-substituted polystyrene as block chains, and proposed in JP-A 10-237143 a block-graft copolymer composition comprising the block-graft copolymer and a polyalkylene oxide. JP-A 10-208545 of the assignee discloses a crosslinked SPE in which polystyrene domain has been chemically crosslinked with a crosslinking agent to form a network structure, in order to protect the polystyrene domain from being dissolved in the polyalkylene oxide added in a large amount to the block-graft copolymer. Additionally, JP-A 10-223042 and 10-245427 of the assignee disclose a self-crosslinking block-graft copolymer whose polystyrene domain can be readily crosslinked merely by irradiating high-energy radiation without a need for a crosslinking agent.
The series of research works completed an SPE which is significantly improved in ionic conductivity and film strength even at high temperatures of 60 to 80.degree. C., with which high-temperature operating batteries at the practically acceptable level can be manufactured on a mass scale.
A particular type of polyalkylene oxide added to the block-graft copolymer exerts some vapor pressure in the temperature region of 60 to 80.degree. C. Also, when the SPE is applied to large-size batteries of the high-temperature operation type, the temperature often exceeds the contemplated temperature region by several tens of centigrade under certain operating conditions. There is a demand to have a battery which is safer because of a wider temperature margin on the high temperature side.